This invention relates to a driver circuit for driving both an inductive load and a non-inductive load connected to the same output node of the driver circuit. Such circuits find use in, for example, automotive applications for driving inductive loads such as relays and non-inductive loads such as lamps from the same output node.
In a typical such combined automotive lamp and relay driver circuit a first driver transistor is provided to drive a relay connected to an output node and a second driver transistor is provided to drive a lamp connected to the output node, the first and second driver transistors being connected in series via the output node and the lamp and relay also being connected in series via the output node.
In use of such an arrangement the first driver transistor is turned "on" and the second driver transistor is turned "off" to drive the relay, whereas the first driver transistor is turned "off" and the second driver transistor is turned "on" to drive the lamp. Under normal operating conditions, logic circuitry ensures that only one of the first and second driver transistors is conducting at any one time. Problems may arise, however, when it becomes necessary to cease driving the relay and begin driving the lamp, i.e. when the first driver transistor is switched from "on" to "off" and the second driver transistor is switched from "off" to "on". When the first driver transistor switches from "on" to "off", the inductance of the relay forces an inductive voltage at the output node in order to keep a constant current through the relay.
The second driver transistor may commonly be provided in the form of a Darlington transistor. Such transistors are commonly provided with internal base-emitter "speed-up" resistors which have associated therewith a parasitic diode component connected across the current electrodes of the transistor. If a transistor with such a diode is used, the diode limits the induced voltage at the output node and allows a dissipation path for the induced current. When the induced energy is dissipated, the second driver transistor turns "on" normally.
Unfortunately, such a diode may not always be present: a high gain Darlington transistor may be needed as the second driver transistor and these typically do not incorporate such "speed-up" resistors and associated diode. Without such a diode the collector-emitter reverse voltage of the transistor is not clamped until the base-emitter junction breaks down in reverse. This is highly undesirable as it may significantly reduce the hFE performance of the transistor in a short time.